On TV Bob always used a 18”x24” double primed canvas. He strongly recommended stretched canvas over a frame, and NOT canvas boards. (He felt the boards sucked up the oil in the paint.

 

Most canvas you buy today are actually triple primed, so it is hard to go wrong.

 

You can use any size canvas you like.  If you want to start painting with smaller canvases, 12x16 is a good size.